The present invention relates to an improved helium detecting unit of the kind employed when searching for leaks in ducts and chambers located in places that are particularly difficult to be accessed and spread over an extended range, far from power supply sources.
For detecting leaks in ducts and chambers of various type and shapes, such as for example the underground pressurized ducts housing telephone cables, there are presently employed techniques using units capable of detecting the concentration of a gas which has been put into the duct the integrity of which has to be tested, and leaks out through cracks that may be present in the duct and can therefore be captured by the detecting unit located outside.
Such units can detect a change in the concentration of a single gas in the gas mixture usually composing the air at ground level, and allow for discovering leaks, if any, due to conduit breaking or cracks with a good accuracy both in respect of the leak location and of the amount thereof.
Units of the above described kind, using helium as a tracing gas, have been disclosed in Italian patents No.s 1 179 600 and 1 224 604 and in EP-A-0352371 in the name of the present Assignee.
The above and other similar devices are equipped with a chamber which is preferably cylindrical, within which a thin quartz capillary is located, being well known that quartz is permeable to helium only when heated to a temperature comprised between 300.degree. and 900.degree. C., wherein one end of the capillary is closed and the opposite end is open and connected to the suction inlet of a UHV (Ultra High Vacuum) pump.
In EP-A-0352371 there is disclosed a helium detector comprising an ion pump and a sniffer probe formed by one or more capillary tubes of silica glass that are closed at one end and connected to the ion pump at the opposite end. A heating filament wound about the capillary tube heats the capillary to a temperature of about 750.degree. C. at which temperature the silica glass becomes permeable substantially to helium only. The ion current drawn by the pump is representative of the helium concentration in the gas mixture to be sampled.
Suitable electronic control systems take care of stopping the heating of the filament when high concentrations of helium are present in order to prevent such gas accumulating within the capillary with a consequent delay of the response to changes of helium concentration in the gas mixture to be analyzed.
The above described devices show anyhow some significant short comings when the units are to be used for checking the condition of ducts or chambers that are located in positions hard to be accessed, such as when they are buried under roadbeds and in sections that are quite long.
In the first mentioned case the weight of the conventional units as well as their construction which is not suitable for an efficient working in presence of dust, mould and other external agents, are an obstacle to the good operativeness of the apparatuses, tire the operator and are subject to frequent clogging with a consequent worsening of the performance.
In the second mentioned case since it is necessary to work over long distances and far from mains power supply sources, auxiliary mobile supply units are to be provided for.
A further shortcoming of the above mentioned units resides in that they have been found subject to an internal overheating after an uninterrupted and prolonged use, particularly in the junction zone between the capillary membrane and the flange provided on the suction inlet of the ion pump, this resulting in a detachment of the UHV (Ultra High Vacuum) seal between the capillary and the flange, thus rendering the detecting unit unserviceable.
A further shortcoming is due to the fact that the above described sensing systems exhibit a large inertia of response when passing from a low helium concentration zone to a high helium concentration zone and vice versa, thus rendering the detection slow and unaccurated.
Finally due to the absence of control devices in the above apparatuses, it may happen that apparatuses that are inoperative or not properly working are employed without the operator's knowledge.